As described in Provisional Patent Applications 60/711,217, 60/711,218, and 60/711,325 incorporated herein by reference, the use of multiple microradios or RFID chips connected to an antenna can in their aggregate provide enough signal strength to be received by a RFID reader.
The integrated circuits that constitute microradios involved in the invention are microscopic in size and are purposely made small to realize cost savings by manufacturing millions of such chips on a single silicon wafer.
The above patent applications describe various ways in which microradios can be coupled to the feed point of an RFID tag antenna. In one embodiment they are embedded in large numbers in a conductive slurry. This is the case where the microradios attach to a single feed point. In the case where the microradios electromagnetically couple to the antenna, the slurry should be non-conductive, preferably with a dielectric constant, a high magnetic permeability or possibly both. When these microradios are deposited in the vicinity of the feed point of the antenna, they couple to the antenna so that the tag can be both interrogated by an RF reader and their response detected.
As described in these patent applications, some of the microradios will be in a proper orientation and polarity to receive interrogating signals and to transmit the requisite information out through the antenna to which they are coupled. Many of the microradios will not be in the appropriate orientation or polarity and are not activated in the programming process.
Assuming there are appropriately activated and programmed microradios or RFID chips coupled to the antenna feed, there is a requirement that when interrogated, all of these microradios transmit at the same time and with the same data so that they work together cooperatively or coherently. It is desired that these microradios when properly coupled and activated will act in unison so that the signals from the tags will add cooperatively and not destructively. If they add destructively the information transmitted will be garbled. If they add cooperatively, the transmission will not be garbled. Also, with cooperative adding there is a quadratic power level enhancement such that the minuscule outputs of the microradios add to provide a more robust tag output signal. Note that a tag made in this fashion will be indistinguishable from a convention single chip tag.
If these RFID chips implement the so-called slotted ALOHA protocol or other similar communications protocols, then these chips would by design transmit at different times and be in different time slots.
The purpose of these protocols is to address the problem of so-called “collisions” between RFID tags that transmit at the same time. This was accomplished by utilizing a pseudo-random number generator to control the time at which a tag would transmit. Upon an interrogation signal, the pseudo-random generators generate different numbers to set different time slots for transmission so that the tags would have a staggered output that would be readable by the reader.
In normal operation, the reader sends out a burst of RF energy and any tag that can receive this energy uses this RF energy to charge up some kind of energy storage like a capacitor that is inside the tag. The electronics in the tags then begin to work and the reader then sends out a query command that says, “OK, is there anybody out there and if you're out there, please respond to me.”
In the simplest case, only one tag is queried and responds to indicate its presence. The reader then acknowledges the existence of the tag and requests the tag information. In one embodiment, the tag then sends back the information, in a simple 96-bit code that would be a unique identifier for the item or product that it is on. The reader then acknowledges receipt of the information and causes the tag to turn off.
As will be appreciated, this standard protocol was devised for the case where one has tags having outputs that collide with each other. In one scenario there might be 50 tags or 100 tags all charged up and ready to go. The reader sends out a query command and, for instance, two of those tags respond in the same time slot. The reader recognizes the collision and asks the tags to try again. Those tags that collided go through a process to pick out a different time slot to respond in and afterwards do not collide with each other.
This change of time slot is done by a pseudo-random number generator. The pseudo-random number generator takes a stored seed number and uses a software program or firmware to generate another number. Each of the tags that have collided has a different seed in them to cause the pseudo-random number generator to output a different time slot number.
After the two colliding tags have picked out new time slots to respond in, the reader comes back and queries the tags again. Then the tag responds at its designated time slot in a normal way, with the other tag then responding in a different time slot. Thus the signals from the two tags are disambiguated.
If one has 50 tags or 100 tags, there is a higher probability of collision and maybe even some probability of multiple collisions. To solve the multiple collisions problem, the above process is invoked sequentially until all 50 or 100 tags are read out.
While the slotted ALOHA protocol is useful in reading out different tags on different items, in item level tagging addressed by the micro miniature radios described herein, if these microradios or RFID chips use the slotted ALOHA protocol and transmit at different times, not only could there be destructive interference, there could be no cumulative n2 signal strength increase due to coherent transmission.